It has long been desired to provide materials for transparent covers which have a low cost per unit, exhibit good luminosity, provide a good security of employment, have durable qualities and which have a good facility of placement.
The use of glass is preferred, over plastic materials, due to its advantages of inalterability and of very large transparence. Unfortunately, glass is badly served by its fragility and thinness.
For obviating this inconvenience, one is able to increase the thickness of the glass. However, this results in a reduction of the transmission of light therethrough, an increase in the price and weight thereof, and, therefore, similar increases relative to the structures of support.
One can equally reduce the volume of the surface of the sheet of glass, However, that involves an increase in bulk of the frame structures and a loss of luminosity. Above all this, such a reduction complicates use of the panels by requiring many more manipulations thereof during construction therewith.
Another solution is to resort to the large surfaces of glass which have been cambered when hot, for benefiting from a better mechanical rigidity. However, the production of hot-cambered glass is very expensive.
Hot-cambered glass is formed from panels of planar glass. The glass is heated near its softening point for performing the operation of placing the glass in the cambered form. This operation is costly in energy. The panels of cambered glass must then be worked and transported in this form, which utilizes excess transportation and storage costs and which greatly increases the risks of breakage.
It is known that a treatment of tempering (thermal or chemical) of glass establishes: on one part, tensions (forces) of permanent compression in the external layers thereof, which has for result a superior resistance to rupture by flexing; and, on the other part, tensions (forces) of pulling the internal layers of the piece of glass, which in case of breakage, results in the piece of glass dividing itself in a large number of fragments, thereby reducing the risks of injury therefrom by laceration.
Sheets of chemically-tempered glass (by a treatment of the diffusion of ions) presents good characteristics of fragmentation (as described above) in case of rupture. Unfortunately, however, sheets of chemically-tempered glass do not present a sufficient resistance to rupture under the effect of the shock of small hard objects which nick the surface of the glass, resulting in penetration. This is due, at least in part, to the fact that the thickness of the layer of glass that is in compression (in the surface of the chemically-tempered glass) is not more than in the order of 50 .mu.m. This defect of chemically-tempered glass is particularly important in cases wherein the sheets of glass have a large area, such as the windshields of automobiles.
French Pat. No. 2,138,711 proposes to remedy this inconvenience of chemically tempered glass by maintaining a sheet of glass in such a way that the tensions (forces) of compression is existent in one of the sides. This side, which is placed in compression, is then utilized as the "exterior face" of the realized article, that is to say, the face of the sheet of glass which is exposed to the elements and other hazards, such as the projections of small hard objects (for example, the exterior face of the windshield of an automobile).
Unfortunately, the technique described in French Pat. No. 2,138,711 exclusively concerns chemically-tempered glass. This technique does not permit, by elastic flexing of a sheet of thermally-tempered planar glass, obtaining a sheet of curved glass whose convex surface is resistant to the impact of small hard objects. Thus, the technique disclosed in French Pat. No. 2,138,711 is not applicable to sheets of glass which have been thermally-tempered in a planar form.
Contrary to chemical-tempering, thermal-tempering involves a thermal modification of the glass which strengthens it throughout its width. This means that its impact resistant qualities are not only found in the surface layers of the glass. Accordingly, even an impact which nicks the surface of the sheet of thermally-tempered glass will not necessarily result in the breakage thereof. Thus, thermally-tempered glass exhibits a superior constraint of rupture over chemically-tempered glass, which increases in function corresponding to its degree of tempering and which is able to withstand constraints of rupture in the order of 200 N/M.sup.2 or more. Elastic-flexing of this thermally-tempered sheet of glass further increases the mechanical strength thereof due to the creation of the forces of compression and pulling described above.
Thus, it can be seen that the goals and use of applications of the present invention is fundamentally different from those of the technique described in French Pat. No. 2,138,711.
Accordingly, it can be seen that there remains a need to realize panels of thermally-tempered glass which benefit from the ease of fabrication and transportation enjoyed by flat sheets of glass (thereby reducing the cost per unit thereof), which can be resiliently-flexed, has a surface extended in relation to its perimeter of framing (thereby providing greater luminosity) and which presents a good security of employment.